a) Field of the Present Invention
The present invention relates to a spindle motor for turning disk-like bodies to be turned, such as magnetic disks.
b) Description of the Related Art
Spindle motors have been used for turning disk-like bodies to be turned, such as magnetic disks. Known as a spindle motor is a fixed-shaft type spindle motor in which a hub is rotatably supported with respect to a shaft via a pair of ball bearings. A rotary-shaft type spindle motor is also known, in which hub and shaft are rotatably supported via a pair of ball bearings with respect to a fixed member of the motor.
In the spindle motor, particles such as oil mists may come outside from the ball bearings, which may spoil the body to be turned. For example, if particles attach onto a magnetic disk and the like, signals cannot be properly reproduced from or recorded on a recording surface of the magnetic disk.
To avoid such situations, the present applicant, then, filed a prior application of a spindle motor having a labyrinth sealing structure with which particles are prevented from escaping outside. The labyrinth sealing structure has been disclosed in Tokkai H8-203203. The spindle motor comprises a pair of ball bearings fitted to a shaft (fixed shaft) standing on a motor frame and a hub rotatably supported with respect to the shaft via the ball bearings. Between an outer ring of the ball bearing placed on the motor frame side and a cylindrical projection vertically standing on the bottom wall of the motor frame, a small gap is configured as a labyrinth sealing structure. A pair of the ball bearings used here are the same in axial dimension.
Prominently used for 2.5-inch and 3.5-inch magnetic disks, apparatus for driving bodies to be turned have rapidly advanced so as to be more compact and thinner. This trend requires spindle motors to be thinner. If the axial dimension of the ball bearing is simply shortened to make a thin motor, however, although a bearing span, a distance between the upper and lower ball bearings, can be sufficiently obtained to some extent depending on fixing positions thereof, the axial dimension of a labyrinth sealing structure configured between the outer ring of the ball bearing and the cylindrical protrusion formed on the frame is shortened, degrading sealing performance against the escape of particles. In addition, if ball bearings having a long axial dimension are used on both upper and lower positions to prevent the sealing performance from being degraded, not only the thickness of the motor is affected, but also the bearing span between the top and bottom ball bearings is shortened. Consequently, rotational precision of the motor is degraded, and the natural frequency of a motor is decreased to easily generate noise.